Daughter boards are coupled to a motherboard via sockets, to expand the functionality of the motherboard. For example, the daughter boards may contain memory modules, or other expansion units to the motherboard.
FIG. 1 illustrates a prior art motherboard including daughter boards coupled to the motherboard via sockets. A motherboard 110 includes a plurality of sockets 120. The sockets 120 are coupled to traces 130 on the motherboard 110. The traces 130 couple signals together, as is known in the art. A daughter card 140 is designed to be inserted into a socket 120. The daughter card 140 includes a number of contacts 150 and 160 on both sides of the daughter card 140. The contacts 150160 of the daughter card 140 make contact with the socket 120. The socket 120 connects these contacts 150 and 160 to the traces 130 on the motherboard 110. The daughter card 140 illustrated has alternating ground contacts 150 and signal contacts 160 on the contact edges of the daughter card 140.
FIG. 2 illustrates a top view of a prior art socket 120. The circles 210, 220 represent the solder holes in the motherboard that receive the pins from the socket. The straight lines 230 represent the contact plane of the daughter board, that is, the area into which the daughter board is inserted. As can be seen, the pins are alternately distal pins and proximal pins. A distal pin is defined as a pin that has its base, the area that contacts the motherboard, far the center of the socket. A proximal pin is defined as a pin that has its base, the area that contacts the motherboard, close to the center of the socket.
The socket includes two parallel rows of pins. The two rows of pins on the socket are arranged to maximize the distance between pins. Thus, each proximal pin faces a distal pin, and each distal pin faces a proximal pin.
The pins are alternately distal pins and proximal pins. Both rows of pins alternate one signal pin 210 and one ground pin 220. The two rows 230 of pins are arranged such that each distal pin in the first row faces a proximal pin in the second row.
FIG. 3 illustrates two cross sections of the prior art socket 120. The first cross section 290 is taken at the cross section line 290 in FIG. 2. The second cross section 295 is taken at the second cross section line 295 in FIG. 2.
Returning to FIG. 3, the first cross section 290 illustrates one distal pin 320 and one proximal pin 310. For one embodiment, the proximal pin 310 and the distal pin 320 are both ground pins 220. The daughter board 330 is inserted such that it contacts both the distal pin 320 and proximal pin 310, one on either side of the daughter board 330. The contacts on both sides of the daughter board 330 at that cross section 290 are ground contacts 220.
The second cross section 295 similarly illustrates one distal pin 350 and out proximal pin 360. In this instance, both the distal pin 350 and proximal pins 360 are signal pins 210. Again, both pins are in contact with the daughter board 330, on either side.
FIG. 4 illustrates a prior art pattern of traces on the motherboard. The circles represent contact areas at which the bases of the pins are attached. The G represents "ground," while the S represents "signal" connections. The first group of contact areas 410, 420, 430, 440 corresponds to a first socket, while the second group of contact areas 450, 460, 470, and 480 corresponds to a second socket. The first signal connection of the first socket is coupled to the first signal connection of the second socket, the second signal connection of the first socket is coupled to the second signal connection of the second socket, and so on. The interconnection is accomplished using traces 130 on the motherboard 110. The traces 130 are routed to maximize the distance between any two traces.
Closely spaced traces 130 may cause interference in some instances, or actual contact between the traces 130 if the traces are imperfectly fabricated. The trace pattern illustrated in FIG. 4 is one prior art trace pattern. As can be seen, the trace pattern has two signal traces 130 crossing between two contact areas 440 at location 490. This is called "two-between routing." Using "two-between routing" may cause interference between the signals carried by either of the traces. Furthermore, having two traces between the contact areas requires the use of thinner traces, to fit the traces between the contact area spacing. Additionally, thin traces have an increased impedance, that does not match the impedance of the signals from the daughter board. Impedance mismatch causes reflected signals, thus degrading signal integrity and limiting operating frequency. Additionally, the connector size is limited by the constraints of printed circuit board design rules associated with "two-between routing."